Pretty sure I remember the report stating both engines 3 and 4 had been feathered (or attempted). Seems likely that #4 had quit and #3 was down on power. Or it could have been pilot error and #3 was feathered on accident.
That much asymmetric thrust while slowing on approach is a deadly combo. Loss of directional authority probably started the rightward yaw, which would likely also induce some right roll too. Any attempt to roll left would only increase drag on the right side as well.
With the right wing low and no power on that side, it's no wonder it took a hard right at impact. It's tragic that a building happened to be right there though.
